Recently mobile communication devices including cellular phones have prevailed in the market, and the devices have been downsized and tagged with a lower price. This market situation demands that the components of the cellular phones be further downsized and the costs thereof be further lowered.
FIG. 11 is a circuit block diagram of a front end of a dual-band cellular phone usable in both Global System for Mobile Communication (GSM) and Digital Cellular System (DCS). This dual-band cellular phone has rapidly gained popularity on a worldwide level particularly in Europe. GSM uses 880-915 MHz as a transmitting frequency and 925-960 MHz as a receiving frequency, while DCS uses 1,710-1,785 MHz as a transmitting frequency and 1,805-1,880 MHz as a receiving frequency.
A transmission signal, applicable to both GSM and DCS, produced in a cellular phone runs through transmitting terminal 102 and is amplified by power amplifier (hereinafter referred to as PA) 105, then the signal is fed into high frequency low-pass filter (hereinafter referred to as LPF) 106.
This high powered transmission signal undergoes LPF 106 as well as transmit/receive switch 107, and arrives at antenna terminal 101. On the other hand, a receipt signal, applicable to both GSM and DCS, received by an antenna runs through antenna terminal 101, switch 107, and is fed into diplexer (branching circuit) 108. The receipt signal is branched into a signal of GSM and a signal of DCS, and both signals undergo respective receiving filters 109 and 110 before arriving at respective receiving terminals 103 and 104.
At this front-end, it is avoided providing PA 105 to respective bands but one PA is shared by both the bands of GSM and DCS with each other for power amplification so that a number of components can be reduced. This one PA for dual-band is going to be a main stream in the market.
In the foregoing structure, a critical point is how to restrain harmonics component developed at the high power amplification done in the PA. For this purpose, if a normal LPF of which cut-off frequency is simply higher than the DCS band is provided, second harmonics of GSM overlap with a transmission band of DCS. Thus the second harmonics of GSM cannot be restrained. Therefore, a cut-off frequency of LPF 106 should be switched to an adequate value responsive to GSM transmission and DCS transmission.
As a result, a conventional LPF is formed of a π type C-L-C filter. These C, L and C comprise a series circuit formed of capacitors 114, 115, another series circuit formed of inductor 118, 119, and still another series circuit formed of capacitors 116, 117 respectively.
In FIG. 12, the conventional LPF has input terminal 111 and output terminal 112. A direct current (DC) is applied to control terminals 113a and 113B for turning on/off diodes 120a and 120b respectively. Inductor 121 and capacitor 122 work as a choke coil and a DC-blocking capacitor respectively.
In FIG. 12, DC voltages applied to control terminals 113a, 113b are set at a High status and a Low status respectively, and diodes 120a, 120b are set conductive and non-conductive respectively. In this case, capacitors 115 and 117 are shorted but inductor 119 is not shorted, thus the cut-off frequency of the LPF is lowered. On the contrary, control terminals 113a, 113b are set at a Low status and a High status respectively, and diodes 120a, 120b are set non-conductive and conductive respectively. In this case, capacitors 115 and 117 are not shorted but inductor 119 is shorted, thus the cut-off frequency of the LPF is raised. As such, use of diodes 120a and 120b allows the LPF to have two cut-off frequencies that can be switched to each other, so that the pass-band is changeable. As a result, the problem discussed above is overcome.
However, according to the foregoing structure, not only at least three diodes are needed, but also components of an ON/OFF circuit for each one of these diodes are needed. Thus it is hard to downsize an entire filter circuit as well as reduce the cost thereof. Further, since numbers of diodes that adversely affect high-frequency characteristics are included in the filter circuit, insertion loss of the LPF increases. This insertion loss influences not only transmission power but also critically a service life of a battery used in a cellular phone. The more diodes are used, the shorter the service life of the battery becomes. It is rather a complicated work to control the ON/OFF of the diodes, and this complication invites some difficulty in manufacturing the filter circuit.